1. Technical Field
This invention relates generally to hydraulic jars for use in drilling equipment and, in particular, to an improved actuating mechanism in which the time required for the jar to trigger is reduced at the initial portion of the waiting time overpull curve, where the waiting time is normally greatest.
1. Description of the Related Art
Drilling jars have long been known in the field of well drilling equipment. A drilling jar is a tool employed when either drilling or production equipment has become stuck to such a degree that it cannot be readily dislodged from the wellbore. The drilling jar is normally placed in the pipe string in the region of the stuck object and allows an operator at the surface to deliver a series of impact blows to the drill string via a manipulation of the drill string. Hopefully, these impact blows to the drill string dislodge the stuck object and permit continued operation.
Drilling jars contain a sliding joint which allows relative axial movement between an inner mandrel and an outer housing without allowing rotational movement. The mandrel typically has a hammer formed thereon, while the housing includes an anvil positioned adjacent the mandrel hammer. Thus, by sliding the hammer and anvil together at high velocity, they transmit a very substantial impact to the stuck drill string, which is often sufficient to jar the drill string free. High velocity is achieved by providing a fluid filled chamber that resists relative movement between the inner mandrel and outer housing for a preselected period of time. Thereafter, the resistance is rapidly and substantially reduced, and the inner mandrel and outer housing are free to move very rapidly relative to one another. Typically, the fluid chamber is pressurized by an initial movement between the inner mandrel and outer housing. Once the pressure of the chamber becomes sufficiently high, movement would cease, but for a metering orifice, which bleeds off fluid at a relatively slow rate until the jar is triggered, and the pressure is rapidly reduced.
For example U.S. Pat. No. 5,085,853, issued Feb. 11, 1992 to Robert W. Evans, describes a double acting drilling jar that contains a fluid chamber and metering orifice. The relationship between the force applied and the time to trigger the jar is exemplarly represented in FIG. 1. That is, the greater the applied force, the shorter the period of time. However, where only a very small force can be applied, the time becomes undesirably long. This can be problematic in at least two situations.
First, where the jar is being assembled into the drilling assembly outside of the well being drilled, it is common for the jar to be manipulated into its collapsed/extended orientation by using only very small applied force. This small force will, of course, require an extended time period to collapse/extend the jar. For example, if the applied force is only about 10,000 lbs, then more than five minutes would be required to complete the task.
Second, when the drill string, with the jar assembled therein, is lowered into the well, it is common for the weight of the drill string below the jar to place sufficient force on the jar that the jar fully extends by the time that the drill bit reaches the bottom of the well. To collapse the jar before drilling, a force is applied to the jar by allowing at least a portion of the weight of the drill string to rest thereon. If a large force is applied for slightly longer than the trigger time of the jar, then the jar will be actuated strongly, driving the drill bit into the bottom of the well bore, and possibly damaging the bit. On the other hand, if only a small force is applied in order to avoid the possibility of damaging the drill bit, then the time required to collapse the jar is inordinantly long.
The present invention is directed to overcoming or minimizing one or more of the problems discussed above.